In on board energy source or storage devices, and particularly in super-capacitor power devices according to the prior art, the protection against fault currents due to insulation losses of the storage module to ground is usually performed by using a high speed circuit breaker on the positive high voltage bus and a fuse on the negative high voltage bus. This solution is critical because relatively high impedance may limit the short circuit current to a value lower than the fuse threshold. The fuse threshold is mandatorily sized over the relatively high thermal current. These kinds of short circuits are particularly dangerous as they may not immediately result in a large current and therefore are less likely to be detected. As a consequence, current can circulate producing damage to the devices or even fire due to overheating.
For the above reasons, a fuse is not suitable for detecting and breaking fault currents of the same magnitude of operating currents under normal conditions, thus relatively low currents can circulate for a long time producing damage to devices or even fire without being detected.
In the following description and in the claims, for the sake of simplicity the term super capacitor power unit or super-capacitor storage unit are used to indicate whichever kinds of on board power devices for storing and supplying electric energy to a load consisting principally, but not in a limiting way, in electric motors of the bogie of a locomotive.
Energy storage systems, such as super-capacitor energy storage systems or other kinds of on board power devices are of great interest for railway vehicles since they allow regulating and reducing energy consumption of such vehicles up to a considerably large extent. Ensuring their safe operation is an important issue, particularly in the field of transportation and specifically in railway vehicles.
Since the components of such electric power storage units are charged to high voltage differences, they are sensitive to losses of insulation to ground so that fault currents can be generated.